It is well known that the injection molding process provides the capability to mass-produce intricate parts in a precise manner. And more and more injection machines have been used for manufacture of micro component parts.
In the prior art, a vertical injection machine consists of an injection unit mounted either with a horizontal or vertical orientation and a clamp system oriented vertically to hold the molds including an upper mold and a lower mold together under force sufficient to resist the pressure exerted by the molten material. The upper mold and the lower mold define a molding cavity therebetween when brought together by a relative vertical movement between them. The molten material can be injected into the mold. After the part cools and solidifies, the mold is opened and the part can be ejected.
Based on the known vertical injection machine, a conventional vertical micro-injection machine includes further a plasticizing unit, an injection unit and a reciprocating screw through which the molten material is fed to an injection cylinder of the injection machine. The molten material is injected by moving the screw forward to force material through a nozzle into the cooled mold cavity. After a desired volume of material in front of the screw has been injected, the axial movement of the screw is stopped. Then the mold cavity is cooled and the injected material is fixed to the desired shape of the part.
Injection molding of micro size parts requires the injection of very small volume of molten material at high precision. The known vertical micro-injection machine uses valves to shut off the molten material and use the movements of screws and/or plungers to meter the exact amount of material to be injected into the mold cavity. The design and manufacture of micro valves that would not trap molten material in a dead zone face many difficulties. Therefore, many new injection machine designs on the market are attempting to resolve these problems.
From US 2002/0020943A1 there is known that the molding machine for molding microparts includes a plasticizing portion operatively connected to an injection portion and a mold portion. A valve member is provided to open and close the connection between the plasticizing portion and the injection portion. The plasticizing portion further includes a plasticizing air cylinder which drives a plasticizing plunger within the plasticizing chamber or bore. However, since the plasticizing plunger is moved in the downward direction to compress the molten material, the air entrapment in the molten material cannot be effectively eliminated, which would influence the qualities of the micro-parts.
U.S. Pat. No. 6,403,010B1 describes a method for injecting plastic or other injectable material into an injection molding tool, which includes: transporting the molten material from a plasticizing apparatus into a dosing apparatus, which has a first fluidic connection to the plasticizing apparatus, and specifically in essentially precisely the quantity that is to be introduced into the injection molding tool; transporting the molten material, essentially in that quantity which is to be introduced into the injection molding tool, from the metering apparatus into an injection apparatus, via a second fluidic connection, while at the same time preventing the molten material from backflowing from the metering apparatus into the plasticizing apparatus; injecting the entire material, which is situated in the injection apparatus, into the injection molding tool by an injection piston, while at the same time preventing the material from flowing from the injection apparatus back into the metering apparatus. In this way, since the molten material is transported from the top down firstly and then transported horizontally to compress the molten material, the air entrapment in the molten material cannot be effectively eliminated, which would also influence the qualities of the micro-parts.